Render issue when the same user is in a group multiple times

[GAsplund]

📝 Describe the bug

The site currently uses only the user ID as a member list ID when viewing a group. However, the same user can be added multiple times.

🔄 To Reproduce

1. Add the same user as multiple posts in Gamma
2. Go to /groups/
3. Encountered two children with the same key

📷 Screenshots

No response

💡 Additional context

This can be resolved if we use (user_id, post_id) as a list ID instead, as (user_id, group_id, post_id) has a unique constraint in Gamma.